1. Field of the Invention
This invention relates to nuclear fuel, and more particularly provides a nuclear fuel rod and a device for placement within the rod cladding at preselected locations to minimize the movement of fuel pellets within the cladding under irradiation and increased temperature.
2. Description of the Prior Art
Nuclear reactors are presently in operation in which the nuclear fuel is contained in fuel elements of various geometric shape, the most typical being in the form of rods. As such, the fuel material is typically enclosed in an elongated cladding of circular cross-section, which is hermetically sealed at its extremities. Common cladding materials include zirconium and its alloys, stainless steel, aluminum and its alloys, and niobium, among others. The fuel within the cladding is typically in the form of solid sintered pellets, each pellet being generally cylindrical in shape. Various fuel materials are utilized, including uranium compounds such as uranium dioxide, plutonium compounds such as plutonium dioxide, thorium compounds, and mixtures thereof. The pellets are axially stacked one atop another, and a typical fuel rod may contain as many as 272 pellets.
It has been found desirable to also provide a void space or plenum within the fuel rods to compensate for the buildup of fission product and other gases released from the fuel pellets during reactor operation. The usual location of such plenums is at the upper portion of the fuel rods, above the fuel pellets. To minimize the potential for damage to the pellets during fuel transport and installation of the nuclear fuel into a reactor core, a restraining device, such as a helical spring, is typically provided in this upper plenum. Such devices may also be utilized to restrict the axial motion of the fuel pellet stack during reactor operation.
Operating experience with such fuel elements has shown, however, that mere incorporation of such restraining devices is insufficient to maintain axial position of the fuel pellets throughout the stack. Fuel element damage has occurred whereby gaps have been formed between pellets in the stack. This phenomenon has been referred to as "fuel densification", as it has been found that the fuel pellets tend to densify and decrease their initial dimensions, thereby allowing relocation within the cladding. A given pellet may, for example, twist and lodge at a given position within the cladding, while those pellets beneath it cumulatively lower their position. Formation of a gap between pellets not only raises concerns over power distribution and control due to the mislocation of the fuel, but also has resulted in local collapse of the cladding at the gaps due to the high external pressure on the rods.
The most common approaches to alleviating these concerns have been directed to the processes involved in the manufacture of the fuel pellets. Varying steps have been taken to provide pellets at a density closer to the theoretical density that had previously been used. It is desirable, however, and has been the history of the commercial nuclear power industry, to provide redundant means to minimize the possibility of any detrimental occurrences. This invention provides such means in the form of a device which is placed at given locations within the fuel rod, which device maintains its axial position, and therefore the position of the pellets, by frictional forces.